A comprehensive study of ultrafast molecular relaxation processes of isomeric meso-(pyridyl) porphyrins (TpyPs) has been carried out by using femtosecond time-resolved emission and absorption spectroscopic techniques upon pumping at 400 nm, Soret band (B band or S2), in 4:1 dichloromethane (DCM) and tetrahydrofuran (THF) solvent mixture. By combined studies of fluorescence up-conversion, time-correlated single photon counting, and transient absorption spectroscopic techniques, a complete model with different microscopic rate constants associated with elementary processes involved in electronic manifolds has been reported. Besides, a distinct coherent nuclear wave packet motion in Qy state is observed at low-frequency mode, ca. 26 cm(-1) region. Fluorescence up-conversion studies constitute ultrafast time-resolved emission spectra (TRES) over the whole emission range (430-710 nm) starting from S2 state to Qx state via Qy state. Careful analysis of time profiles of up-converted signals at different emission wavelengths helps to reveal detail molecular dynamics. The observed lifetimes are as indicated: A very fast decay component with 80 ± 20 fs observed at ∼435 nm is assigned to the lifetime of S2 (B) state, whereas being a rise component in the region of between 550 and 710 nm emission wavelength pertaining to Qy and Qx states, it is attributed to very fast internal conversion (IC) occurring from B → Qy and B → Qx as well. Two distinct components of Qy emission decay with ∼200-300 fs and ∼1-1.5 ps time constants are due to intramolecular vibrational redistribution (IVR) induced by solute-solvent inelastic collisions and vibrational redistribution induced by solute-solvent elastic collision, respectively. The weighted average of these two decay components is assigned as the characteristic lifetime of Qy, and it ranges between 0.3 and 0.5 ps. An additional ∼20 ± 2 ps rise component is observed in Qx emission, and it is assigned to the formation time of thermally equilibrated Qx state by vibrational cooling/relaxations of excess energy within solvent. This relaxed Qx state decays to ground as well as triplet state by 7-8 ns time scale. The femtosecond transient absorption studies of TpyPs in three different excitations at S2 (400 nm), Qy (515 nm), and Qx (590 nm) along with extensive global and target model analysis of TA data exclusively generate the true spectra of each excited species/state with their respective lifetimes along with microscopic rate constants associated with each state. The following five exponential components with lifetime values of 65-70 fs, ∼0.3-0.5 ps, ∼20 ± 2 ps, ∼7 ± 1 ns, and 1-2 μs are observed which are associated with S2, Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states, respectively, when excited at S2, and four (Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) and three (hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) states are obtained when excited at 515 nm (Qy) and 590 nm (Qx), respectively, as expected. The TA results parallel the fl...

Understanding of exciton dynamics in semiconductor quantum dots (QDs) is of great importance due to their immense application potential on various photonic devices. In this work, we demonstrate hot electron transfer (HET) from higher excited states of cadmium telluride quantum dots (CdTe QDs) to tetrakis(4-carboxyphenyl)porphyrin (TCPP) and ultrafast electron transfer from photoexcited TCPP to CdTe QDs in newly prepared CdTe QD−tetrakis(4-carboxyphenyl)porphyrin nanocomposites (CdTe QD−TCPP NCs), where TCPP is noncovalently attached to CdTe QDs, by employing steady state, time-resolved emission, and femtosecond transient absorption spectroscopic techniques. The observation of efficient quenching of the photoluminescence (PL) of CdTe QDs with little/negligible change in photoluminescence (PL) decay profiles in nanosecond time regime (TCSPC method) of CdTe QD−TCPP NCs predominantly indicates static interaction between these two interacting species, CdTe QD and TCPP. Excitation wavelength and excitation intensity dependent femtosecond transient absorption studies of CdTe QDs and CdTe QD−TCPP NCs confirm the hot electron transfer (HET) from higher excited states (Σ − /1P(e)) to TCPP. Analysis of growth kinetics of band-edge or 1S bleach amplitude of band-edge bleach signal strength at the initial stage after excitation allows us to estimate 30−40% quantum efficiency (Φ HET ) of HET when CdTe QD−TCPP NCs are excited at higher excited state (∼390 nm). In contrast, no electron transfer was observed when CdTe QD−TCPP NCs are excited in the vicinity of the band-edge (1S) exciton. In another instance, femtosecond transient absorption studies upon 630 nm excitation at the Q-band of TCPP in CdTe QD−TCPP NCs suggest the occurrence of ultrafast electron transfer (<250 fs) from photoexcited TCPP to CdTe QDs in CdTe QD−TCPP NCs. The charge separation and charge recombination dynamics in CdTe QD−TCPP NCs are explored in detail. The fundamental understanding of this "to and fro" photoinduced electron transfer dynamics in CdTe QD−TCPP NCs opens up new possibilities to design an efficient light-harvesting system based on inorganic−organic hybrid systems.

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